The momentum-transfer cross sections for positronium (Ps)–gas molecule scattering in the low energy region are determined by analysing the thermalization process of ortho-positronium (o-Ps) in various gases. The momentum distributions of Ps in Ne, H2, CH4, N2, C2H4, n-C4H10, i-C5H12 and neopentane are measured as functions of mean lifetime, by using the one-dimensional angular correlation of annihilation radiation method. The average energies obtained for o-Ps with mean lifetimes of 3–90 ns are analysed, to yield the momentum-transfer cross sections for the Ps–gas molecule scattering. The cross sections thus determined are σm = (20 ± 8) × 10−16 cm2 for Ne, (16 ± 3) × 10−16 cm2 for H2, (17 ± 6) × 10−16 cm2 for CH4, (37 ± 10) × 10−16 cm2 for N2, (35 ± 8) × 10−16 cm2 for C2H4, (100 ± 30) × 10−16 cm2 for n-C4H10, (110 ± 50) × 10−16 cm2 for i-C5H12 and (180 ± 60) × 10−16 cm2 for neopentane, in the Ps energy range below 0.3 eV. The Ps is thermalized mainly through elastic scattering with diatomic and polyatomic molecules, as in the case of scattering with inert gases.

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